Histidine kinases (HKs) are abundant among prokaryotes and have been characterized in fungi and plants, although not yet in animals. These enzymes regulate diverse processes, including adaptation to osmotic stress and virulence of plant and animal pathogens. Here, we report the cloning, characterization and phylogenetic analysis of HwHHK7A and HwHHK7B, HK genes from the fungi Hortaea werneckii, a proposed model system for studying salt tolerance in eukaryotes. The two HwHhk7 isoforms are 96.7% identical in amino-acid sequence and have a typical eukaryotic hybrid HK domain composition. On the bases of the conserved sequence of the H box, they are classified into the group VII ascomycete HKs. For the HwHhk7B protein, the autokinase activity was demonstrated in vitro. The salt-responsive expression of the HwHHK7 genes and the increased osmotolerance of a wild-type Saccharomyces cerevisiae strain expressing the HwHHK7B gene lead us to speculate that these newly identified HKs have roles in osmosensing.
"Similarly, the WiHog1B isoform is a fully functional Hog1-like kinase in W. ichthyophaga, while WiHog1A can only partly complement Hog1 function in S. cerevisiae (Konte and Plemenitas, 2013). Interestingly, as supported by high amino-acid-sequence conservation, the stress-dependent gene expression profiles and complementation of the S. cerevisiae hog1D mutant, the MAPKs HwHog1A (Turk and Plemenitas, 2002; Lenassi et al., 2007) and the newly identified HwHog1B are fully functionally redundant, similar to homologues in Z. rouxii. This might be the consequence of the relatively recent whole genome duplication in H. werneckii, which has yielded two nearly identical copies of every protein-encoding gene (Lenassi et al., 2013) that still have not undergone functional specialisation. "
"This discovery is in line with our previous studies of several individual genes from H. werneckii that were present in two copies (Gorjan and Plemenitaš, 2006; Lenassi and Plemenitas, 2007; Fettich et al., 2011). In most cases, the expression of both of the gene copies is salt dependent, although their expression profiles differ (Lenassi and Plemenitas, 2007). It may well be that as a consequence of this whole genome duplication, H. werneckii can benefit from the potential advantages of large genetic redundancy, even though it is formally in a haploid stage (i.e., it is not a diploid that has resulted from the mating of two strains with opposite mating types, which would regain the haploid stage with meiosis before the next mating event; see below). "
[Show abstract][Hide abstract] ABSTRACT: Molecular studies of salt tolerance of eukaryotic microorganisms have until recently been limited to the baker's yeast Saccharomyces cerevisiae and a few other moderately halotolerant yeast. Discovery of the extremely halotolerant and adaptable fungus Hortaea werneckii and the obligate halophile Wallemia ichthyophaga introduced two new model organisms into studies on the mechanisms of salt tolerance in eukaryotes. H. werneckii is unique in its adaptability to fluctuations in salt concentrations, as it can grow without NaCl as well as in the presence of up to 5 M NaCl. On the other hand, W. ichthyophaga requires at least 1.5 M NaCl for growth, but also grows in up to 5 M NaCl. Our studies have revealed the novel and intricate molecular mechanisms used by these fungi to combat high salt concentrations, which differ in many aspects between the extremely halotolerant H. werneckii and the halophilic W. ichthyophaga. Specifically, the high osmolarity glycerol signaling pathway that is important for sensing and responding to increased salt concentrations is here compared between H. werneckii and W. ichthyophaga. In both of these fungi, the key signaling components are conserved, but there are structural and regulation differences between these pathways in H. werneckii and W. ichthyophaga. We also address differences that have been revealed from analysis of their newly sequenced genomes. The most striking characteristics associated with H. werneckii are the large genetic redundancy, the expansion of genes encoding metal cation transporters, and a relatively recent whole genome duplication. In contrast, the genome of W. ichthyophaga is very compact, as only 4884 protein-coding genes are predicted, which cover almost three quarters of the sequence. Importantly, there has been a significant increase in their hydrophobins, cell-wall proteins that have multiple cellular functions.
Frontiers in Microbiology 05/2014; 5:199. DOI:10.3389/fmicb.2014.00199 · 3.99 Impact Factor
"In B. cinerea, the HOG1-like MAP kinase is not required for fungicide sensitivity even though it is negatively regulated by the " two-component " HSK. Furthermore, the salt-tolerant yeast species, Hortaea werneckii, copes with osmotic stress using a Group VII HSK- HOG1 pathway . ose studies indicate that the HSK- HOG1 signaling pathways can be operated in very different regulatory mechanisms in various species. "
[Show abstract][Hide abstract] ABSTRACT: The production of host-selective toxins by the necrotrophic fungus Alternaria alternata is essential for the pathogenesis. A. alternata infection in citrus leaves induces rapid lipid peroxidation, accumulation of hydrogen peroxide (H2O2), and cell death. The mechanisms by which A. alternata avoids killing by reactive oxygen species (ROS) after invasion have begun to be elucidated. The ability to coordinate of signaling pathways is essential for the detoxification of cellular stresses induced by ROS and for pathogenicity in A. alternata. A low level of H2O2, produced by the NADPH oxidase (NOX) complex, modulates ROS resistance and triggers conidiation partially via regulating the redox-responsive regulators (YAP1 and SKN7) and the mitogen-activated protein (MAP) kinase (HOG1) mediated pathways, which subsequently regulate the genes required for the biosynthesis of siderophore, an iron-chelating compound. Siderophore-mediated iron acquisition plays a key role in ROS detoxification because of the requirement of iron for the activities of antioxidants (e.g., catalase and SOD). Fungal strains impaired for the ROS-detoxifying system severely reduce the virulence on susceptible citrus cultivars. This paper summarizes the current state of knowledge of signaling pathways associated with cellular responses to multidrugs, oxidative and osmotic stress, and fungicides, as well as the pathogenicity/virulence in the tangerine pathotype of A. alternata.
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